Field of the Invention
The present invention relates to a semiconductor device, and in particular relates to a wide-bandgap semiconductor device using a wide-bandgap semiconductor.
Description of the Background Art
For power saving of a power electronics device such as an inverter, it is necessary to reduce a power loss of a switching device, such as an insulated gate bipolar transistor (IGBT) and a metal oxide semiconductor field effect transistor (MOSFET).
Since a power loss depends on a conduction loss and a switching loss of a switching device, in order to reduce these losses, wide-bandgap semiconductor devices using a wide-bandgap semiconductor such as silicon carbide (SiC) and gallium nitride (GaN) have been developed.
When a power MOSFET is used as a switching device, a return current (freewheel current) can be passed through a parasitic diode of the power MOSFET (hereinafter referred to as a body diode). It is known that using a body diode enables downsizing or omission of a freewheel diode placed in parallel to the power MOSFET. This has been applied to power converter circuits.
An SiC semiconductor device has a problem in that a bipolar operation using p-type and n-type carriers leads to expansion of a crystal defect due to carrier recombination energy and increase in resistance. This problem also occurs when passing the freewheel current through the body diode. Increase in a power loss and occurrence of a malfunction caused by increase in on-state resistance of a power MOSFET have posed a problem.
WO 2014/148294 discloses a method of passing a large current through a SiC-MOSFET to apply current stress and expanding a crystal defect for screening. A screening method according to this prior art employs a method of expanding a crystal defect of a body diode to saturation and then determining the degree of change in a forward resistance by setting a temperature of a bipolar device in a chip state at 150 to 230° C. and continuously passing a forward current of a current density of 120 to 400 A/cm2 through the bipolar device.
In a SiC-MOSFET, improving reliability of a body diode is important for securing stability of device operation and assuring reliability in a market. As described in WO 2014/148294, applying electric power to a body diode in a chip state, expanding a crystal defect, and then measuring and evaluating a forward characteristic make it possible to secure reliability of the body diode and secure stability of the device operation.
However, it is difficult, by the screening method of WO 2014/148294, to fully expand a crystal defect in a termination region provided outside an active region of a SiC-MOSFET by applying current stress. A possible cause thereof is that recombination energy is unlikely to reach the crystal defect in the termination region provided outside the active region and assumed stress is not applied to the crystal defect, and that a crystal defect resulting from such a macroscopic defect as a carrot-like defect does not expand to saturation.
In the latter case, when the macroscopic defect is present within the active region of the SiC-MOSFET, determination can be made by withstand voltage characteristic evaluation of a test process. However, when the macroscopic defect is present outside the active region and recombination energy necessary for defect expansion does not reach the defect by applying current stress to the body diode, while the body diode is used as a freewheel diode, it cannot be evaluated whether the crystal defect outside the active region expands and affects the body diode, indicating that the screening method of WO 2014/148294 cannot fully secure reliability of the body diode.